Hot-melt adhesives are thermoplastic materials which are solid at room temperature and which, on heating, become viscous liquids. These viscous liquids are applied to a first substrate and then the substrate is covered with a second surface. On cooling, adhesion is obtained between the substrate and the second surface. The assembly time is the period during which the adhesive which has been applied to a substrate, which is at room temperature, remains tacky, that is to say the interval of time during which it is possible to apply the second surface and, on cooling, to obtain adhesion between the substrate and the second surface.
Once this assembly time period has been exceeded, it is no longer possible to obtain sufficient adhesion between the substrate and the second surface.
These adhesives are denoted by the abbreviation HMA (hot-melt adhesives). Adhesives having an infinite assembly time are useful for self-adhesive labels or adhesive tapes which are used at room temperature. Depending on the nature of the adhesive, it is possible to obtain more or less strong adhesions and, for example, to detach and reattach the label. Bonding is achieved by pressure at room temperature. The adhesive is deposited on the substrate (for example, label or tape) while hot. These adhesives are denoted by HMPSA (hot-melt pressure-sensitive adhesive). The present invention relates to this type of adhesive. They are also sometimes denoted as self-adhesive hot-melt compositions.
According to the prior art, see, for example, Ullmann's Encyclopedia of Industrial Chemistry, 5th edition (1995), Vol. A 26, p. 659-660, HMPSAs are preferably based on SIS (styrene/isoprene/styrene) or SBS (styrene/butadiene/styrene) block copolymers.
Mention has been made, in U.S. Pat. No. 5,373,041, relating to HMPSAS, as disadvantages of SBSs and SISs, of their instability towards heat and towards U.V. radiation, as well as of their poor resistance to oils. Provision has thus been made, in this patent, to replace SBSs and SISs by copolymers composed of 60 to 70% (by weight) of ethylene, 30 to 40% of n-butyl acrylate and from 0 to 5% of (meth)acrylic acid, which can be neutralized by metal ions.